Grinding devices of the general type described supra have been known for quite a while and are used in a plurality of applications. Exemplary embodiments are a so called roller mill and a so called vertical mill.
A roller mill typically includes two horizontal rolling cylinders which rotate opposite to one another, wherein both rolling cylinders have a minimum distance from one another or are in contact with one another at a knuckle where they form a grinding portion. The material to be ground or grinding material is introduced from a top side of the grinding portion between the two roller cylinders, wherein the individual particles of the grinding material stream pass through the grinding portion and are ground. Grinding devices of this type are used for example for grinding grain. An exemplary embodiment can be derived among others from WO 2009/067828 A1.
Vertical mills, however, are mills in which the grinding material is placed onto a horizontally arranged grinding table which rotates about a vertically oriented axis. In an outer circumferential edge portion of the grinding table in which the grinding material is collected based on the impacting centrifugal forces, typically plural so called roller mills are arranged whose rolling elements are formed by vertically standing rollers, whose rotation axis is horizontally oriented. The grinding portion in this type of mills is between a respective bottom side of the roller and the grinding table wherein due to the rotation of the grinding table about the vertical axis the grinding material is continuously moved along under the roller. Thus, the roller is pressed in a direction towards the grinding table, wherein the weight of the roller and also external pressing forces that are applied by the contact pressure device become effective. Under this pressure that is imparted by the roller onto the grinding material the grinding material is ground. Vertical mills of this type are typically used in the concrete industry. An exemplary embodiment can be derived among others from DE 10 2008 046 921 A1.
In particular the latter vertical mills that are known in the art have a basic problem in that they tend to enter an instable vibration condition which is commonly referred to as “rumbling”. In this condition the grinding device is vibrating which causes the roller and the grinding table to move relative to one another in a vertical direction, this means the roller is at least lifted by the grinding bed formed by the grinding material and can even lift off and subsequently presses or impacts on the grinding bed again. Here dynamic forces in an order or magnitude of several mega Newton [MN] can be at work so that the vertical mill can be damaged quite easily. For example a roller jacket which circumferentially envelops the roller is subject to a very high load in this instable vibration condition.
During operation of such grinding devices accordingly there is a long felt need to avoid these load conditions. Therefore monitoring systems are typically installed which detect particular operating parameters of the mill which eventually shall be used for drawing reverse conclusions with respect to a critical load. As a result there is the problem that shut downs and thus economically disadvantageous idle times of the mill occur due to anticipation of an impending resonance. Furthermore it happens from time to time that the described “rumbling” of the mill occurs in spite of these monitoring strategies.
The recited DE 10 2008 046 921 A1 relates among other things to this problem and attempts to monitor the grinding device so that critical load conditions are detected reliably and early, wherein the dynamic forces impacting the rollers shall be detected in particular frequency ranges and a shutdown of the entire grinding device shall be performed when reaching a threshold value.
In another document, EP 2 408 565 B1, the problem of rumbling mills is also discussed. The document describes a vertical mill whose contact pressure device is configured as an “open system”. This means that the contact pressure device which is formed by the at least one hydraulic cylinder and the at least one gas spring additionally includes at least one hydraulic pump through which an oil pressure in the at least one hydraulic cylinder and/or the at least one gas spring can be continuously adapted. In particular EP 2 408 565 B1 described that the effect of the hydraulic pump can load a lower pressure chamber of the hydraulic cylinder with pressure which causes the corresponding roller mill to “lift off”, this means that at least one contact pressure of the roller mill is reduced, optionally even a contact between the roller mill and the grinding bed is completely lost. This shall help to quiet the resonating mill system.
A disadvantage of the latter system is on the one hand side the complexity of the open pressure system which requires operating a hydraulic pump. On the other hand side the disclosed device as such is not free from disadvantageous vibration conditions (“rumbling”) but only provides a system which shall resolve the rumbling in a particularly simple manner should it occur.
Regardless, EP 2 408 565 B1 also provides a prevention strategy with regard to rumbling wherein the prevention strategy is based on a pressure adaptation of the rolling mills based on the effect of the hydraulic pump, wherein a pressure adaptation is used in the opposing pressure chambers of the hydraulic cylinders. This control system, however, is complex and slow since a pressure buildup by the hydraulic pump as a counter measure against a critical resonance that builds up takes a rather long time period, thus an entry of the mill system into resonance probably cannot be prevented in a timely manner.
Therefore a system which reliably prevents the risk of rumbling is not known in the art at all.